The specific excitation of mercury isotopes by photochemical means is well established. See Webster, C. R. and R. W. Zare, "Photochemical Isotope Separation of Hg-196 by Reaction with Hydrogen Halides," J. Phys. Chem., 85:1302 (1981), the teachings of which are incorporated herein by reference.
Mercury lamps are commonly used as the excitation source of Hg isotope-specific photochemical reactions. To be successful, the photochemical separation of a single isotope requires the spectral bandwidth of the exciting mercury lamp or laser source to be sufficiently narrow to excite only the specific isotope of interest. This specificity depends, in part, on the spectral bandwidth of the source. The rate and extent of separation of the particular isotope from the feedstock can be strongly dependent on the intensity of the radiation emitted from the mercury lamp.
The vapor equilibrium pressure of the Hg used in current mercury lamps strongly affects the spectral linewidth and intensity of the light which is emitted from these lamps. Previously described systems used for this purpose are not able to adequately control the Hg vapor pressure inside of the lamps. This is due to the fact that the lamp cold spot, (i.e., the lowest temperature region within the lamp) is not well established. The cold spot temperature is critical as it determines the equilibrium vapor pressure within the lamp.
After start-up of a typical photochemical lamp, many hours of lamp operation may be required to fix the cold spot region. During this transition time, a definite vapor pressure of Hg within the lamp envelope is not attained. This variation in the vapor pressure of the mercury within the lamp can cause disturbances in the linewidth and intensity of resonance radiation emitted. Thus, undesirable isotopes of Hg can be stimulated and the rate of separation of the desired isotope of mercury can be affected. Further, without identifying the location of the cold spot, it may be difficult to accurately monitor the Hg vapor pressure.
Microwave powered, electrodeless lamps have previously been used in photosensitized mercury isotope separation processes. One such lamp is described in U.S. Pat. No. 4,746,832 issued May 24, 1988, to Grossman et al., entitled "Controlling the Vapor Pressure of a Mercury Lamp." These microwave lamps have a mercury vapor envelope which is partially inserted into a microwave source. When microwave radiation is generated, it excites the vapor mixture within the envelope, thereby causing the lamp to emit light. Lamps of this type require accurate control of cold spot temperature regions and have power coupling characteristics which have exhibited great sensitivity to the overall lamp length. Additionally, the power input for microwave lamps is strongly dependent on the relative positions between the microwave cavity acting as the microwave source and the lamp envelope for conditions at which photochemical separation processes are operated.
The vapor equilibrium pressure in microwave lamps can be controlled by establishing and controlling two temperature zones within the lamp. The first of the two temperature zones establishes a cold spot and the second zone is maintained at a temperature equal to or greater than the first zone. In this manner, greater control of the temperature within the lamp can be obtained. This allows the equilibrium vapor pressure of the Hg within the entire lamp to be regulated. However, lack of accurate cold spot control and difficulty in increasing lamp length with constant power coupling throughout the discharge length have been found with this type of lamp. Consequently, a variety of variables must be carefully monitored to achieve a constant equilibrium pressure of the Hg vapor and the desired spectral emission.
There have been additional methods reported for enriching isotope .sup.196 Hg by isotope-selective photooxidation of mercury vapor using microwave lamps, (see, for example, M. Desnoyer et al., J. Ch. Phys. Tome, 60:14-16 (1963); J. P. Morand et al., J. Chim. Phys., 65:2058-2068 (1968); J. P. Morand et al., Energie Nucleaire, 10:362-366 (1968); G. Mueller et al., Iosotope Practice, 17(5):200-205 (1980); and G. Mueller et al., German Patent No. 124,144 (1977)). Each of the microwave lamps described in the above references requires the use of a single water cooling loop. This design displays large power losses due to the coupling of a large amount of microwave power into the water loop.